1. The Field of the Invention
The present invention relates to systems and methods for attaching two or more members together. More specifically, the present invention relates to a novel system and method for securely and easily mounting a rounded member, for example, an airbag inflator, to an interior surface of a vehicle.
2. The Relevant Technology
Many methods are currently available for fastening two or more parts together as part of an assembly. Flat parts, such as steel beams, struts, and the like can typically be comparatively easily fastened together through the use of fasteners, adhesives, welding, or a similar method. Adjoining flat surfaces provide an even, simple interface for the attachment.
However, rounded members, such as bars, pipes, pressure vessels, and the like present greater attachment problems. Round, convex surfaces often require the use of a corresponding concave surface to provide an attachment interface. Thus, many implements, such as conventional clamps and the like, that are useful for attaching two flat parts together, cannot be used for rounded parts.
The attachment problem is further multiplied when the rounded member is subject to high stress. In the case of pressure vessels, for example, a comparatively thin wall is subject to high stress from a pressurized internal fluid. The walls of such vessels are typically manufactured to have a uniform thickness and a comparatively uniform curvature so that stresses are evenly distributed throughout the wall. Thus, significant deformation or piercing of the wall is to be avoided. Holes, in particular, are problematic even if they do not extend fully through the wall of the pressure vessel, because stresses tend to concentrate at holes. As a result, cracks often begin forming at holes, and propagate outward from the hole. Although thinner wall sections are not as critical as holes, they are also often failure points because of stress concentration.
As a result, the number of methods that can be used to attach a rounded member under considerable stress to another member is very limited. Fasteners that require holes, such as screws, bolts, rivets, and the like, are clearly undesirable. Welding also has a tendency to weaken the underlying material, and requires that the joint to be welded be accessible to the welding equipment.
Conventional press fitting, or xe2x80x9cinterferencexe2x80x9d fitting, is an attachment process by which a member is attached to another member or a fixture by friction. xe2x80x9cFrictional engagementxe2x80x9d refers to two surfaces that are pressed together such that friction keeps them from sliding relative to each other. xe2x80x9cInterferencexe2x80x9d refers to a geometric state in which one part blocks motion of another part; in an interference fit, one or both parts are deflected to make the relative motion possible.
In order to provide an interference fit, a protrusion in one member is typically inserted into a cavity in another, and the cavity is dimensioned slightly smaller than the protrusion. The cavity must then be stretched, and the protrusion compressed, in order to fit together. A considerable amount of radial pressure between the protrusion and the cavity results, so that the protrusion is held within the cavity by frictional force. Often, the protrusion, the cavity, or both may be tapered so that the protrusion can be gradually forced into the cavity.
The force required to force the protrusion into the cavity is generally proportional to the force required to withdraw it. In order to create an attachment that will withstand a desired axial (along the axis of symmetry of the rounded member) tension, a commensurate degree of compression may need to be applied to insert the protrusion into the cavity. However, in circumstances in which torsion, or rotational force, is to be coupled with the tension, a lower amount of tension may be required to withdraw the protrusion.
One such application in which it is desirable to rigidly mount a rounded member is for automotive safety restraint devices. The inclusion of inflatable safety restraint devices, or airbags, is now a legal requirement for many new vehicles. Airbags are typically installed in the steering wheel and in the dashboard on the passenger side of a car. In the event of an accident, an accelerometer within the vehicle measures the abnormal deceleration and triggers the ignition of an explosive charge. Expanding gases from the charge fill the airbags, which immediately inflate in front of the driver and passenger to protect them from impact against the windshield. Side impact airbags have also been developed in response to the need for similar protection from impacts in a lateral direction, or against the side of the vehicle.
The explosive charge is typically located in an inflator, which often takes the form of a cylindrical metal pressure vessel designed to contain the explosion and channel the resulting gases into the airbag. Since the inflator contains explosive materials, it is very important that it be firmly fastened to an interior surface of the vehicle. The inflator typically has a cylindrical central portion with roughly hemispherical end caps. Thus, the problems described above in connection with attachment of rounded members generally, apply to inflators. Additionally, Department of Transportation regulations restrict the use of any welded joints in motor vehicles. Generally, attachments in motor vehicles, particularly attachments related to safety systems, must be strong enough to withstand the operating vibrations of the vehicle as well as potential impacts.
Furthermore, known attachments are typically not adaptable to inflators of different sizes. Airbag sizes may vary from one vehicle to the next; consequently, an airbag manufacturer may need to be able to make and install several different inflator sizes. With most known attachment systems, each inflator size would require its own specially-sized attachment. The need to pair each size with an associated attachment assembly has increased the time and expense required for inflator installation.
Consequently, it would be an advancement in the art to provide a method and apparatus for attaching a rounded member to another member without welding. More specifically, it would be an advancement in the art to enable the attachment of a rounded member such as an inflator to a comparatively flat surface such as a vehicle surface.
The method and apparatus should preferably be easily carried out with a minimum of equipment. Thus, the method and apparatus should preferably provide a comparatively large holding force with a comparatively small attachment force. Preferably, the method and apparatus should be capable of maintaining attachment even under combined axial and torsional loads. The method and apparatus should also be usable to attach rounded members with a wide range of sizes, without the need to design and use different attachment hardware with each size. Furthermore, the method and apparatus should be simple and inexpensive to implement.
The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available systems and methods for attaching rounded members. Consequently, the present invention provides a novel system and method for attaching a rounded member, particularly to a flat surface. Although the following disclosure provides the example of an airbag inflator, the method and system disclosed herein may be used with any rounded member.
An inflator attachment may comprise an inflator, a flange, and a spacer. The inflator typically takes the form of a cylindrical pressure vessel with roughly hemispherical ends. The inflator is located within a suitable compartment of a vehicle, such as a passenger side dashboard compartment. The inflator provides pressurized gas to an airbag, either through a conduit, or through direct passage of air into an opening of the airbag from an opening, or diffuser, positioned in a first end of the inflator. The inflator may be attached to a vehicle surface within the compartment at a second end of the inflator, so as to maintain the inflator in a cantilevered, suspended position within the compartment.
In the alternative to the cantilevered configuration, the first end of the inflator may be attached in similar fashion to a bolt. Thus, the diffuser may have threads sized to engage an opening of the vehicle surface, or a nut used in combination with an opening to secure the first end. Other attachment methods such as crimping may also be used to secure the first end. If desired, the airbag can then be folded into the compartment with the opening of the airbag facing the inflator.
Either form of attachment may be facilitated by attaching the flange to the second end of the inflator. In the case of a cantilevered attachment, the second end of the inflator may actually be attached to a vehicle surface through the use of the flange. If the first end is attached, the flange may simply be used as a shoulder to maintain the second end properly positioned with respect to the first, and to support the inflator against axial and torsional forces that may be applied during installation.
Preferably, the flange has a shank and a web. The shank has a tubular configuration large enough to fit with clearance around the inflator. The web may then comprise a perpendicular, disc-like extension with a flat mounting surface that can be mated to the vehicle surface within the compartment. The web may be affixed to the vehicle surface through fastening, adhesive or chemical bonding, welding, or any other suitable method.
The spacer may comprise a compressible, split-ring design that can slide relatively easily over the inflator in its uncompressed configuration. The spacer is positioned between the inflator and the flange, and is preferably dimensioned so as to be compressed between the spacer and the inflator. If desired, the spacer may be constructed of a material harder than the inflator and/or the flange, so as to create indentations in the inflator and/or flange during assembly. The indentations then serve to keep the spacer in position with respect to the inflator and the flange. The edges of the spacer are effectively held by interference within the indentations so that no axial motion of the spacer is possible. If desired, the spacer may be formed with indentations or other shaped features to increase the amount of interference and thereby increase the resistance of the interference fit to axial force.
In addition, to the extent that the spacer comprises any radial irregularities, such as gaps (as in a split ring), protrusions, or the like, the indentation is shaped accordingly. Thus, the interference of the indentation with the radial irregularities effectively precludes rotation of the ring in response to torsional forces acting on the inflator during installation of the inflator or operation of the vehicle. In order to enhance resistance against torsional force, the spacer may have a configuration different from a split ring. For example, a series of curved blocks, separate or connected by ring sections, may be utilized. Alternatively, ridges parallel to the axis of symmetry of the spacer may be formed on the inside and/or the outside of the spacer.
In addition to the enhanced resistance to axial and torsional force, the present invention provides a number of distinct assembly advantages. Notably, in certain embodiments, a comparatively small assembly force may be used, even though the force required to remove the flange from the inflator remains large.
Initially, the spacer may be slid into position around a circumferential portion of the second end of the inflator. A tubular support designed to fit around the inflator with clearance, and within the flange with clearance, may then be positioned in abutting relation to the ring. The flange is preferably constructed with a tapered inside diameter, in which a larger portion is sized to fit over the spacer with clearance, and a smaller portion is small enough to interfere with the spacer, while still fitting over the inflator with clearance. The larger portion may be positioned proximate the web.
The flange may thus be slid over the inflator, with the web and the larger portion of the tapered inside diameter leading, from the first end of the inflator toward the second end. The larger portion of the tapered inside diameter slides over the spacer, and the smaller portion of the tapered inside diameter comes into contact with the spacer. The support and inflator may then be held firmly in place while the flange is forced further toward the second end of the inflator. The narrowing inside diameter of the flange effectively forces the spacer inward, so that the spacer firmly engages the inflator, and the flange firmly engages the spacer.
Alternatively, in embodiments in which the spacer is harder than the inflator and the flange, the spacer may simply be pressed into the inflator to form the indentation prior to inclusion of the flange. This may be accomplished through the use of an external press, thermal contraction, or any other known method. The flange may then be assembled onto the spacer and inflator as described above, or by another method. For example, the flange may be created in modular portions and assembled around the spacer/inflator arrangement, or the flange may be heated, positioned around the spacer and inflator, and then allowed to contract and cool.
Such a method of assembly provides numerous advantages over known interference fit operations. Deformation of the inflator is limited to a comparatively narrow, circumferential portion of the inflator. Additionally, in certain embodiments, a comparatively small force must be applied over only a comparatively small distance to bring about secure engagement of the inflator, spacer, and flange. The tapered inside diameter of the flange strongly resists outward motion of the inflator from the vehicle surface because outward motion of the inflator tightens the interference fit.
Additionally, particularly where the spacer is an expandable structure, such as a split ring, the same spacer and flange can be fitted on inflators with different diameters. The tapered interior diameter of the flange permits the flange to be installed over spacers with a range of sizes; the flange simply engages the spacer at a different location within the flange.
These and other advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.